1. Field of the Invention
This invention relates to an electromagnetic drive device for a shutter and, more particularly, to an electromagnetic drive device for a programmed electromagnetic shutter in which a coil is disposed within a magnetic field produced by a permanent magnet. Shutter stop blades are arranged to be driven by a Lorentz force exerted on the coil when the coil is energized with an electric current.
2. Description of the Prior Art
For forming a programmed electromagnetic shutter with an electromagnetic drive device of a moving coil type, the prior art includes a method wherein a printed substrate board, having a coil provided thereon by etching, is mounted on a sector ring and opposes a magnet to have each shutter stop blade driven thereby. This method is employed in a device called a printed rotor type electromagnetic drive device. With the electromagnetic drive device arranged in this manner, it has been impossible to have a rotation shaft positioned at the center of rotation of a moving part, such as the sector ring or the coil. Hence, the device is greatly affected by a frictional force produced by rotation of the moving parts. To increase the responsiveness of the device, the mass of each moving part must be as small as possible. In view of this, the printed rotor type electromagnetic drive devices in general have light-weight moving parts to improve responsiveness. The use of light-weight moving parts, however, results in the shortcoming that the absolute value of the output torque of rotation cannot be large. An electromagnetic shutter composed of such an electromagnetic drive device tends to have the shutter stop blades thereof make an erroneous action due to a force exerted from outside the camera. With an electromagnetic drive device of this type employed, it has been difficult to impart sufficient reliability to the operation of an electromagnetic drive shutter. Therefore, in the printed rotor type electromagnetic drive device, it has been impossible to obtain a large output torque by increasing the number of turns of the coil. In order to obtain an electromagnetic drive device that is sufficiently invulnerable to disturbance from outside by increasing the output torque, it is necessary to have a wound coil of a mass larger than a certain given value pivotally supported at the center of rotation of the coil; to have the coil opposed to a magnet; and to have the rotation of the wound coil transmitted to the shutter stop blades through a lever or the like. It is a recent trend to reduce the size of cameras. In the case of the above-mentioned electromagnetic drive device, which is known by the name of a meter type electromagnetic drive devices the device is required to be of a shape permitting incorporation thereof within a compact camera without difficulty. It is difficult to provide a meter type electromagnetic drive device which can be incorporated in a compact camera which has a sufficiently large output torque for driving the shutter stop blades without being affected by an outside force.